The researchers, led by the University of Sydney's Dr Louise Hardy, surveyed 2,750 11- to 15-year-old school students from New South Wales on their cardiovascular fitness and screen use and concluded that a child's level of aerobic fitness is strongly associated with their level of 'small screen time' and general sedentary behaviour.

The findings mark the first physiological evidence supporting the guidelines put forward by the American Academy of Pediatrics (AAP) in 2001, which were based on expert opinion and also suggested a two hour cap of sedentary behaviour for children.

"The AAP's advice was essentially drawn from prudence. Our findings represent the first physiological link between more than two hours small screen recreation and lower aerobic fitness," said Dr Hardy.

The researchers also found that girls were more likely to experience lower fitness as their sedentary behaviour increased compared with older boys whose fitness was less affected.

"We think these boys, who would be over fifteen years, probably have developed enough muscle mass which allows them to 'sit and be fit'," Dr Hardy said.

"Still, it is important to understand most children need to exchange excess 'sitting time' for 'active time' and limit any time in front of a screen to less than two hours a day in order to maintain fitness levels," she added. "No screen days would be even better."

Unlike previous studies, the researchers also included activities like reading, text messaging, doing hobbies and even homework in order to achieve a more inclusive representation of sedentary behaviour amongst children than just time spent in front of a television or computer.

The team's research has been published in the February edition of the American Journal of Preventive Medicine.